1. Field of the Invention
The present invention concerns counterflow heat exchangers and finds particular application in those that are components of mass-produced distillers.
2. Background Information
Distillation is probably the single most effective approach to purifying water. But it has historically been too costly for widespread use. Distillation requires that the water evaporate. Without energy recovery, the energy of vaporization alone would cost something on the order of fifteen to twenty cents per gallon or more. Theoretically, that cost can be reduced by recovering and reusing the heat of vaporization. For most small-scale distillation applications, though, the equipment available until now has not had the capability of recovering enough heat to make distillation affordable.
But more-recent designs have shown that small, low-component-cost distillers can distill water with high efficiency. For example, a fire-plug-sized distiller based on U.S. patent application Ser. No. 10/870,018 of William H. Zebuhr for a Blade Heat Exchanger has been constructed that can produce distilled water at an operating cost of less than half a cent per gallon.
In that design, the influent to be purified is heated to near its saturation temperature and sprayed onto heat-exchange surfaces in the evaporation chamber of a rotary heat exchanger. Such a heat exchanger uses centrifugal force to keep the liquid film on its heat-exchange surfaces much thinner than surface tension would ordinarily permit. As a consequence, those surfaces transfer heat of vaporization to the influent very efficiently.
A compressor draws the resultant vapor from the evaporation chamber, leaving contaminants behind. The compressor raises the vapor's pressure and delivers the higher-pressure (and thus higher-saturation-temperature) vapor to the rotary heat exchanger's condensation chamber. In that chamber, thermal communication with the evaporation chamber results in the vapor's condensing into a largely contaminant-free distillate, surrendering its heat of vaporization in the process to the influent in the evaporation chamber. The rotary heat exchanger thereby recovers the heat of vaporization efficiently.
Such a system also needs to recover the heat that raised the influent to the temperature at which it is delivered to the rotary heat exchanger, and this can be achieved readily in a counterflow heat exchanger. In such a heat exchanger the condensed but still high-temperature distillate is cooled by being brought into thermal communication with the incoming influent across thermally conductive dividers. In the process the distillate heats the influent nearly to the desired evaporation-chamber temperature. (Further heat increase is typically accomplished by, e.g., using the influent to cool the compressor motor.)
As was stated above, such a distiller can be made small, so it has the potential to be manufactured inexpensively. But achieving that potential requires that the distiller's components be assembled with speed and simplicity. And a problem that arises in this connection is how to seal the counterflow heat exchanger's thermally conductive dividers.
Sealing is a problem because the divider's area should be relatively high in comparison with the area of the counterflow heat exchanger's other conduit-defining surfaces (which contribute to cost and undesired heat transfer). The high-surface-area requirement dictates that the divider be convoluted rather than flat. In one design, for example, the divider results from folding a flat sheet multiple times in such a manner that each fold cooperates with its neighbor folds to define longitudinally extending influent and distillate channels. A consequence of such a design is that, instead of having a flat sheet's basically one-dimensional cross section, the divider cross section undulates, forming alternating end openings for adjacent channels. The welding, soldering, and other approaches conventionally used to seal such end opening would add significantly to a small distiller's cost of production.